This invention relates generally to ultrasonic flowmeters and more particularly to a noninvasive ultrasonic flowmeter which utilizes signal phase modulation to determine fluid volumetric flow rate in a closed conduit or pipe.
There are three basic categories of non-invasive ultrasonic flowmeters which are used for the measurement of average fluid flow velocity in closed conduits. These are: (1) time-differential, (2) Doppler shift, and (3) cross-correlation.
The time-differential type of flow meter employs the principle that an ultrasonic pulse travelling across the conduit with a component of its velocity along the direction of flow will traverse the conduit in slightly less time than a pulse travelling in the opposite direction, that is, with a velocity component opposing the flow direction. The time-differential type flowmeter requires very sophisticated timing circuits to measure the time intervals which in some cases may be less than a nanosecond. Furthermore, the transducers must be arranged with one upstream of the other. This arrangement introduces refraction of the ultrasonic pulse because it must pass through the conduit wall, the fluid, and the opposite wall at an oblique angle.
The Doppler-shift type of ultrasonic flowmeter utilizes the presence of traces of impurities in the fluid to scatter the incident ultrasonic signal. When the signal is thus scattered it becomes frequency shifted (i.e. Doppler-shifted) in proportion to the velocity of the scattering impurity. However, the constant of proportionality depends on the refractive angle of the transmitted ultrasonic beam.
The cross-correlation type of ultrasonic flowmeter is the only one of the three types which avoids the problems of refraction of the signal. In this type of ultrasonic flowmeter an ultrasonic beam is transmitted diametrically through the conduit and fluid by transducer pairs at each of two locations separated by a distance L. The disturbances caused to the transmitted beams by fluid turbulence and inhomogeneities are obtained by phase demodulation of the received signal. The time delayed upstream signal is then cross-correlated with the downstream signal as a function of the delay time T. The delay for which the cross-correlation is a maximum is taken as a measure of the time required for turbulent fluctuations to be convected the distance L between the pairs of ultrasonic transducers. The flow velocity is then computed as L/T.
Although the cross-correlation type avoids the problems associated with refraction of the ultrasonic beam, it utilizes very complex circuits which make it more expensive than either of the other two types. Also, the complex circuitry required hinders the development of small, portable units for survey use.